Increasing bandwidth demands on fiber-optic networks have driven network designs toward more closely spaced optical wavelength channels and also toward higher transmission bit rates. Spectral characteristics of these lightwave systems are indicators of transmission performance. For closely spaced dense wavelength-division-multiplexed (DWDM) systems, the limited resolution of conventional diffraction grating-based and autocorrelation-based optical spectrum analyzers (OSA) reduces the ability to measure spectral features between DWDM channels.
As an alternative to conventional OSAs, optical heterodyne detection systems can be utilized to monitor DWDM systems. In an optical heterodyne detection system two lasers operate at optical frequencies whose difference is within the frequency range of a microwave spectrum analyzer that is used to detect a generated heterodyne beat signal. One of the lasers is typically referred to as the local oscillator (LO).
FIG. 1 is a depiction of a prior art optical heterodyne detection system 100. The optical heterodyne detection system 100 includes an input signal 102, an input waveguide 104, a local oscillator signal 106, a local oscillator waveguide 108, an optical coupler 110, an output waveguide 118, a photodetector 112, and a signal processor 116. The principles of operation of optical heterodyne detection systems are well known in the field of optical heterodyne detection and involve monitoring the heterodyne term that is generated when an input signal is combined and mixed with a local oscillator signal.
However, in system 100, the resulting heterodyne beat signal from the photodetector 112 is normally bandwidth limited to provide high spectral resolution. As a result, due to the random difference in relative phasing of the optical input signal and the local oscillator signal, different responses are possible. In consequence, amplitude accuracy of the heterodyne optical spectrum analyzer is degraded by the phase noise. Moreover, intensity noise is a problem affecting prior art heterodyne optical spectrum analyzers. In some instances, the intensity noise renders measurement results inaccurate or unusable. As a consequence, amplitude accuracy of the heterodyne optical spectrum analyzer is degraded by the intensity noise.